Nanostructured deposition and devices

ABSTRACT

Ion conducting solid electrolytes are constructed from nanoscale precursor material. Nanocrystalline powders are pressed into disc structures and sintered to the appropriate degree of densification. Metallic material is mixed with 0 to 65 vol % nanostructured electrolyte powders to form a cermet mix and then coated on each side of the disc and fitted with electrical leads. The electrical conductivity of a Ag/YSZ/Ag cell so assembled exhibited about an order of magnitude enhancement in oxygen ion conductivity. As an oxygen-sensing element in a standard O 2 /Ag/YSZ/Ag/N 2  set up, the nanocrystalline YSZ element exhibited commercially significant oxygen ion conductivity at low temperatures. The invention can be utilized to prepare nanostructured ion conducting solid electrolytes for a wide range of applications, including sensors, oxygen pumps, fuel cells, batteries, electrosynthesis reactors and catalytic membranes.

RELATED APPLICATIONS

[0001] This application is a continuation of copending U.S. Ser. No.09/251,313 entitled “Nanostructured Solid Electrolytes and Devices”,filed Feb. 17, 1999, which is a continuation of U.S. Ser. No.08/739,257, filed Oct. 30, 1996, which is a continuation-in-part of U.S.Ser. No. 08/730,661, entitled “Passive Electronic Components fromNano-Precision Engineered Materials,” filed on Oct. 11, 1996, which is acontinuation-in-part of U.S. Ser. No. 08/706,819, entitled “IntegratedThermal Process and Apparatus for the Continuous Synthesis of NanoscalePowders,” now issued as U.S. Pat. No. 5,851,507 on Dec. 22, 1998, andU.S. Ser. No. 08/707,341, entitled “Boundary Layer Joule-Thompson Nozzlefor Thermal Quenching of High Temperature Vapors,” filed concurrently onSep. 3, 1996, now issued as U.S. Pat. No. 5,788,738 on Aug. 4, 1998.These applications and patents are all commonly owned with the presentapplication, and are all incorporated by reference herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention pertains in general to ion conductors and toprocesses for the synthesis of ion conducting solid electrolytes. Inparticular, the invention relates to the use of nanoscale powders forthe preparation of nanostructured oxygen ion conducting electrolytes.

[0004] 2. Description of the Prior Art

[0005] Solid electrolytes are materials through which ion species canmigrate with low energy barriers. Table 1 outlines some examples ofion-conducting structures, representative materials, and the ionsconducted. These materials are of critical commercial importance toelectrochemical devices, components and processes. Illustrativeapplications include sensors, batteries, fuel cells, ion pumps, membranereactors, catalysis, and metallurgy. REPRESENTATIVE MATERIALS IONCONDUCTED Stabilized ZrO₂ System, Stabilized Bi₂O₃ O²⁻ System, Ceria,Perovskites Beta-Alumina, NASICON Systems Na⁺ AgI, RbAg₄I₅ Ag⁺Rb₄Cu₁₆I₇Cl₃ Cu⁺ Li₃N, Li₂S-SiS₂-Li₃PO₄ System, Organic Li⁺ PolymerSystems, LISICON Systems

[0006] As a specific example, stabilized zirconia is a known conductorof oxygen ions. Accordingly, its properties are utilized in variousfields of technology, such as in oxygen sensors for fuel-air ratiooptimization of automobiles and furnaces, in oxygen pumps for solidstate oxygen separation, in solid-oxide fuel cells for noiseless andclean power generation from chemical energy, and in catalytic membranereactors.

[0007] The oxygen-ion conduction properties of stabilized zirconia usedin a typical oxygen sensor are well understood based onelectrochemical-cell theory. When placed between two compartmentscontaining a reference gas and an analyte oxygen gas at differentpartial pressures, stabilized zirconia functions both as a partitionbetween the two compartments and as an electrochemical-cell electrolyte.Under ideal conditions, the open-circuit emf (E₀) of the cell is givenby the known Nernst equation: $\begin{matrix}{{E_{0} = {\frac{RT}{4F}\quad \ln \quad \left( \frac{{PO}_{2}^{Ref}}{{PO}_{2}} \right)}},} & (1)\end{matrix}$

[0008] where T is the absolute temperature of the cell; PO₂ ^(Ref), andPO₂ are the partial pressures of oxygen in the reference and analytecompartments, respectively; R is the universal gas constant; and F isFaraday's number.

[0009] According to this equation, any difference in partial pressure ofthe oxygen across the two faces of the oxygen-conducting electrolytegenerates an electromotive force that depends on the temperature andpartial-pressure ratio of the oxygen in the two compartments of thesystem. In order to generate Nemstian response in sufficiently shorttimes, the temperature of stabilized ZrO₂ needs to be high (above 700°C.), which results in relatively high power requirements and inincreased equipment mass and size, need for insulation, and attendantsealing problems. These considerations often produce unsatisfactoryperformance or affect the commercial viability of products based onstabilized ZrO₂ technology.

[0010] The inherent reasons for the high-temperature requirement and thecorresponding performance problems of present-day oxygen ion conductingelectrolyte based devices can be traced to the reaction mechanism of thecell and the microstructure of the sites where the reaction occurs.Referring to FIG. 1A, a schematic drawing of a ZrO₂ sensor cell 10 isillustrated, where the stabilized zirconia is modeled as a solidelectrolyte membrane 12 between a first compartment 14, containing areference oxygen atmosphere at a predetermined partial pressure PO₂^(Ref), and another compartment 16 containing an analyte gas with oxygenat a different partial pressure PO₂. The two sides of the stabilizedzirconia non-porous solid electrolyte 12 are coupled through an externalcircuit connecting an anode 18 and a cathode 20 made of porous metal,such as silver. The anode 18 is the cell electrode at which chemicaloxidation occurs and the electrons released by the oxidation reactionflow from it through the external circuit to the cathode. The cathode 20is the cell electrode at which chemical reduction occurs. The cellelectrolyte 12 completes the electrical circuit of the system byallowing a flow of negative ions O₂ ⁻ between the two electrodes. Avoltmeter 22 is provided to measure the emf created by the redoxreactions occurring at the interfaces of the electrolyte with the twooxygen atmospheres.

[0011] Thus, the key redox reaction of the cell occurs at the pointswhere the metal electrode, the electrolyte and the gas meet (illustratedin the inset of FIG. 1 as the “triple point” 24). At each such site onthe surface of the electrolyte 12, the redox reaction is as follows:

O₂ (gas)+4e⁻→2O₂ ⁻.  (2)

[0012] Since the reaction and the electrochemical performance of thesensor depend on the redox kinetics, the cell's performance is a strongfunction of the concentration of triple points. In other words, anelectrode/electrolyte/electrode cell with as many triple points aspossible is highly desirable [see Madou, Marc and M. Morrison, ChemicalSensing with Solid State Devices, Academic Press, Boston (1989)]. In thecase of an oxygen cell with a ZrO₂ solid membrane and silver electrodes,this requirement corresponds to maximizing the triple points on eachside of the PO₂.Ag′/ZrO₂/Ag″PO₂ ^(Ref) system.

[0013] Another cause of poor performance of oxygen-sensor cells can beexplained with the help of complex-impedance analysis. Referring toFIGS. 2a and 2 b, a complex impedance diagram for a ZrO₂ sensor isshown, where the impedances of the bulk, grain boundary and electrodeare illustrated in series to reflect their contribution to the ionicconduction at each triple point. It has been shown that the conductiveperformance of electrolytes at temperatures below 500° C. is controlledby the grain boundary contribution to the overall impedance. Thus, forsignificant improvements of the conductivity at low temperatures, it isnecessary to significantly minimize the grain-boundary (interface)resistance.

[0014] In summary, oxygen ion conducting devices based onstabilized-zirconia electrolyte have two problems that can be traced tomaterial limitations. First, the electrolytes have high impedance;second, the concentration of triple points is relatively low. Theseproblems are common to solid oxygen-conducting electrolytes inparticular and solid electrolytes in general, and any improvement inthese material characteristics would constitute a significanttechnological step forward. The present invention provides a novelapproach that greatly improves these aspects of ion conducting solidelectrolytes.

SUMMARY OF THE INVENTION

[0015] One of the objectives of this invention is to enhance theion-conductivity of solid electrolytes by preparing nanostructured solidelectrolytes.

[0016] Another objective is to reduce the electrolyte thickness with theuse of nanostructured precursors of solid electrolytes.

[0017] A further objective is to enhance the concentration of triplepoints in the ion conducting devices by using nanostructured precursorsand materials.

[0018] Yet another objective of the invention is to utilize the uniqueproperties of size confinement in solid electrolyte and electrode grainswhen the domain is confined to less than 100 nanometers.

[0019] Another objective of this invention is an oxygen-conductingelectrolyte material with low-impedance oxygen conductingcharacteristics.

[0020] Another objective of the invention is an oxygen ion conductingdevice with a very high density of triple points.

[0021] Another goal is a process and materials that reduce the cost ofmanufacture of products that incorporate oxygen-ion conductors.

[0022] Yet another goal is a process and materials that reduce the costof operation of products that incorporate oxygen-ion conductors.

[0023] Finally, another goal is a process that can be readilyincorporated with conventional methods for manufacturing productscontaining ion-conducting electrolytes.

[0024] Various other purposes and advantages of the invention willbecome clear from its description in the specification that follows andfrom the novel features particularly pointed out in the appended claims.Therefore, to the accomplishment of the objectives described above, thisinvention comprises the features hereinafter illustrated in thedrawings, fully described in the detailed description of the preferredembodiments and particularly pointed out in the claims. However, suchdrawings and description disclose only some of the various ways in whichthe invention may be practiced.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is a schematic drawing of a ZrO₂ solid electrochemical cellwhere the stabilized zirconia is modeled as a solid electrolyte membranesandwiched between a first compartment containing a reference oxygenatmosphere at a predetermined partial pressure and another compartmentcontaining an analyte gas with oxygen at a different partial pressure.

[0026]FIGS. 2a and 2 b are a complex impedance diagram for a ZrO₂sensor, where the impedances of the bulk, grain boundary and electrodeare illustrated to reflect their contribution to the ionic conduction ateach triple point shown in FIG. 1.

[0027]FIG. 3 is an X-ray diffraction pattern of the nanoscale yttriastabilized zirconia precursor used to form an electrolyte membraneaccording to the invention.

[0028]FIG. 4 is a graph of total conductivity versus temperature of9-YFSZ nanozirconia prepared by the process of Example 1 andmicron-based YFSZ material.

[0029]FIG. 5 is a voltage-versus-temperature graph of a nanostructuredoxygen sensor manufactured with the material prepared in Example 1. Thesymmetrical response about the abscissa relates to switching the gasesfrom one face of the sensor to the other.

[0030]FIG. 6 is a flow chart of the process for preparing a yttriastabilized bismuth oxide (YSB) nanopowder from nitrates by a solutionmethod.

[0031]FIG. 7 is an X-ray diffraction pattern of the nanostructured YSBproduct of Example 2.

[0032]FIG. 8 illustrates Nyquist plots of impedance spectra of cellswith the nanocomposite electrodes of Example 3 in comparison with a cellwith pure Ag electrodes.

[0033]FIG. 9 is the voltage response in oxygen of the sensor of Example3 as a function of temperature in comparison with that of a conventionalYSZ sensor.

[0034]FIG. 10 shows electrode resistances of pure Ag electrode andnanocomposite electrodes as a function of temperature, as determinedfrom the impedance spectra measured in air.

[0035]FIG. 11 shows a comparison of the ionic conductivity ofnanostructured YSB electrolyte with YSZ electrolyte.

[0036]FIG. 12 is a flow diagram of the steps of deposition over asupporting substrate according to known vapor deposition processesapplicable to the manufacture of nanostructured electrolytes accordingto the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0037] This invention is based on the recognition that the ionconductivity of polycrystalline solid electrolytes at moderate andnear-ambient temperatures is mainly controlled by the conductivity ofgrain boundary and the concentration of triple points. The inventionfurther notes that the engineering of grain boundary resistance andtriple points in solid electrolyte devices is limited by the electrolytethickness and electrode characteristics, respectively, which in turndepend on grain size of the precursors and the material used in themanufacture of electrolytes and electrodes for solid ion conductors ingeneral, and solid oxide oxygen sensors, solid oxide oxygen pumps andsolid oxide fuel cells in particular. These limitations constitute aninherent obstacle to achieve significant technological improvements.

[0038] The finest powders currently available for commercial use consistof particles with sizes in the order of several microns. For example,the YSZ powders that are presently used to produce oxygen sensors havean average grain size of about 1 to 3 microns. Since the number oftriple points occurring within a given area at the interface with theoxygen atmosphere is necessarily limited by the number of electrodegrains distributed within that area, the grain size in the electrode isvery important for maximizing redox-reaction sites. Similarly, since weknow that the impedance of the system is reduced by electrolytethickness, it follows that thinner electrolytes produced from smallergrains would produce lower impedance. Accordingly, the heart of thisinvention consists of using nanosize materials in the manufacture ofelectrolytes for these applications.

[0039] The current inability to improve the performance of solid ionconductors is a result of the inability of prior-art processes toeconomically reduce powder size of precursor materials beyond themicron-size range. Accordingly, the present invention is based on thework disclosed in commonly-owned U.S. Pat. Nos. 5,851,507 and 5,788,738which provide a viable vehicle for manufacturing, nanoscale powderssuitable for the present invention. Material having with physicalproperties as produced by the process and apparatus described therein isa necessary ingredient for practicing this invention on a commercialscale.

[0040] As defined in the art, submicron powders are materials havingaverage grain size below 1 micrometer. Of critical interest for thisinvention are nanoscale powders and nanostructured layers of ceramicsand electrodes. Nanoscale powders (nanopowders) are submicron powderswith average grain size less than 100 nanometers (preferably with astandard deviation of less than about 25 nm) and with a significantfraction of interfacial atoms. Accordingly, reference to nanoscalepowders in this disclosure is intended to refer to powders with thosecharacteristics.

[0041] Submicron layers are layers having thickness less than 1micrometer. Of particular interest to this invention are nanostructuredlayers which are defined specifically as layers with thickness, ormicrostructure, or both, confined to a size less than propertyconfinement size (positively less than 1 micron, preferably below 100nm). Accordingly, reference to nanostructured layers in this disclosureis also intended to refer to layers with those characteristics.

[0042] As discussed in the copending applications, it is known thatwithin these size ranges a variety of confinement effects occur thatdramatically change the properties of the material. The idea of thisinvention then is to build ion conducting solid electrolytes frompowders whose grain size has been confined to dimensions less than 100nanometers. The size confinement effects in nanometer scale can confinefundamental processes to band-gap and quantum confined states which inturn can dramatically change the properties and performance of theresulting solid electrolyte. This insight can be implemented as devicesprepared with one dimensional quantum dot and nanocluster composite withthe dot size less than 100 nm (preferably less than 10 nm), as quantumwires with diameter less than 100 nm (preferably less than 10 nm), asquantized and nanoscale films with film thickness less than 100 nm, asnanostructured layers and pellets with microstructure less than 100 nm,and as a combination of these. In summary, another aspect of theinvention concerns the preparation of solid electrolyte and electrodesthat are nanostructured.

[0043] Nanostructured ion conducting electrolytes prepared fromnanostructured materials have grain sizes spatially confined to lessthan 100 nanometers; a significant fraction (20-60%) of their atoms isinterfacial, and exceptional interactions occur between the constituentdomains. Therefore, nanostructured oxygen-conducting electrolytes can beexpected to have very high concentrations of interface area which canassist rapid and low-temperature densification of ion conductingelectrolytes. The nanoscale powder can also enable dramatic reduction inlayer thicknesses as discussed in co-pending applications. Furthermore,since nanostructure provides higher density of surface area, the densityof triple points at the electrolyte-electrode-gas interface can also besignificantly enhanced using nanostructured-electrolyte/electrodeinteractions. Given low resistance and high triple-point concentration,nanostructured electrolytes and electrodes can be used to achieve higherion conductivity and electrochemical activity. This is of particularinterest when an ion conducting device has to operate at near ambienttemperatures. This general design principle is applicable to all, solidion conductors based on ion defect structure, two dimensional layeredstructure, three dimensional network structure, vitreous structure,α-AgI type structure, and composites prepared using these structures.Illustrative examples include, without limitation, oxide ion conductorssuch as stabilized zirconia, stabilized ceria, stabilized bismuth oxide,perovskites, LISICON, NASICON, and β-alumina.

[0044] The following examples illustrate different ways of reducing thepresent invention to practice.

EXAMPLE 1

[0045] A stock solution was prepared from ZrOCl₂.8H₂O and 9 mol % Y₂O₃in water, and diluted with denatured ethanol. The solution was chilledto 0° C. and then slowly added to a continuously stirred basic solutionof ammonium hydroxide that was also maintained at 0° C. Precipitation ofwhite precursor powder was instantaneous. The precipitate solution wassuction filtered, and the gelatinous filter cake was washed in denaturedethanol three times. The loose powder so generated was dried quicklywith mild heating at 100° C. to remove water and ethanol, and calcinedto 500° C. in air to form nanocrystallites with grain size of about 5.8nm, standard deviation of 1.1 nm. This precursor material consisting of9 mole-percent yttria stabilized zirconia (YSZ) nanoscale powders wasexamined using, an X-ray diffractometer (XRD). A typical XRD pattern forthe 9 mole powders so produced is illustrated in FIG. 3, which showsthat the ZrO₂ is stabilized cubic phase. In order to determine theaverage particle size of the powders, the widths of strong, low orderpeaks of XRD pattern were analyzed using Scherrer's method. The averageparticle size of the powders according to this analysis was found to beabout 4.5 nanometers. The particle size was also verified bytransmission electron microscopy (TEM). The results suggested a particlesize of 5.8 nanometers.

[0046] The nanoscale 9 mole % yttria stabilized cubic zirconia powderswere pressed into 3 mm diameter discs (0.15 gram weight) and sintered tohigh densities (preferably more than 90% of theoretical density formechanical strength, over 95% being preferred). The sample disks weresintered at low temperatures (1,150 to 1,250° C., yielding more than 95%density) and for short duration (6 to 24 hours) to minimize graingrowth. We found that YSZ nanopowders readily sintered to fulltheoretical densities at about 1,200° C. in 17 hours, which representsignificantly milder and less expensive processing conditions thanpresently necessary. Careful control of the sintering profile and timecan further reduce the sintering temperature and time. The cylindricaldiscs were examined under XRD and the post-sintered mean grain size byScherrer analysis was found to be about 83 nm, confirming that the discswere nanostructured. The two ends of the cylindrical discs so producedwere then coated with a cermet paste consisting of a mix of silver andnanoscale yttria stabilized zirconia powder (about a 50-50 wt % mix,corresponding to a 35 Ag-65 YSZ vol % mix). Then platinum leads wereattached to the cermet layer.

[0047] The samples were placed in a furnace and their impedance wasmeasured in air as a function of increasing temperature with acomputerized impedance analyzer. A standard 40 mV AC bias and frequencysweep range of 5 Hz to 13 MHz were used. As illustrated in FIG. 5, theresults so obtained suggest that nanostructured oxygen-conductingelectrolytes, referenced by n, exhibit almost an order of magnitudehigher oxygen-ion conductivities at lower temperatures when comparedwith base-line electrolytes, referenced by μ (i.e., conventionalmicron-powder based oxygen-conducting electrolytes). It is noted thatneither the baseline nor the nanostructured electrolytes representoptimal performance. For additional electrochemical and electrocatalyticperformance evaluation, the Ag/YSZ/Ag cell was tested as a sensor andoxygen pump. For sensor/fuel-cell experiments, oxygen containing gas waspassed over one face of the sensor and nitrogen was passed over theother face of the sensor. The emf response as a function of temperaturewas measured. As shown in FIG. 5, the results indicate that the sensorsignal for each gas combination is linear with temperature, confirming aNernst-type behavior. For electrosynthetic oxygen generation and pumpingapplications, CO₂ was passed over one face while nitrogen was passedover the other face. FIG. 5 shows that the oxygen-conducting electrolyteexhibited oxygen pumping properties at low temperatures.

EXAMPLE 2

[0048] Bismuth nitrate (Bi(NO₃)₃.5H₂O) and yttrium nitrate(Y(NO₃).6H₂O), were used as precursors for preparing nanosized yttriastabilized bismuth oxide (YSB) powder via solution co-precipitation.FIG. 6 shows a flow chart of the co-precipitation processing steps usedin this example. After precipitation, the precipitate solution wassuction filtered, and the gelatinous filter cake was washed in acetoneto minimize agglomeration of ultrafine powder due to hydrogen bonding.The loose powder so generated was next dried with mild heating to removewater and acetone. Then the powder was calcined in air at 500° C. for 2hours. XRD showed that calcine schedule resulted in a single cubic YSBphase (see FIG. 7). The volume averaged crystallite size of the powderfired at 500° C. was determined to be 12.5 nm by analyzing thebroadening of the (111) diffraction peak and applying Scherrer'sformula. The YSB nanopowder was characterized in terms of morphology andparticle size by transmission electron microscopy (TEM). The averageparticle size was estimated to be about 15 nm, which is in goodagreement with the result obtained from XRD analysis.

[0049] The nanopowders were uniaxially pressed at 50,000 psi into greenpellets of 12.5 mm in diameter and 1 mm in thickness. The pressingprocess consisted of initially lubricating the die with a die lube,followed by the weighing of an appropriate amount of powder, insertingthe powder in the die, uniaxially pressing to the desired pressure,holding at that pressure for 30 seconds, and then slowly releasing thepressure over 15 seconds. Subsequently, the pellet was forced out fromthe die. No binder was added for the forming, process. It was found thatfrom the nanopowder the electrolytes can be sintered with greater than96% of theoretical density at temperatures ranging from 850 to 950° C.In contrast, YSB electrolytes made from micron-sized powder aretypically sintered at temperatures greater than 1,000° C. It is knownthat the primary driving force for densification of ceramics is thereduction of free surface area at high temperatures. The very small sizeof the YSB nanopowder, therefore, has a very large driving force fordensification; thus, the required sintering temperature can besignificantly reduced relative to commonly used micron-sized powders.This is an important manufacturing advantage of this invention.

[0050] The concept of the invention is also applicable to improve theperformance of electrodes for ion conducting materials. These electrodesshould have high electrical conductivities, high catalytic activities,adequate porosity for gas transport, good compatibility with theelectrolyte, and long-term stability. In order to achieve high catalyticactivities, it is preferred that the electrode be highly porous, so thatit retains a large number of active sites for electrochemical reactions,i.e., the triple points. Ag has been studied as an electrode materialbecause it is known to have high electrical conductivity and highcatalytic activity for oxygen reduction and evolution. However, pure Agelectrodes readily densify during processing and operation, resulting ina dense electrode with little porosity. In order to reduce the electroderesistance, the teachings of this invention were used to preparenanocomposite electrodes from Ag and nanostructured powders of the ionelectrolyte material.

EXAMPLE 3

[0051] YSB electrolyte pellets, 19 mm in diameter and 0.9 mm inthickness, were sintered from green pellets of 25 mm in diameter and 1.2mm in thickness. The pellets were ground and polished to a thickness of0.7 mm to provide a suitable electrolyte substrate. A separate compositepaste of 79 vol % Ag and 21 vol % YSB was prepared by mixing nanopowderof YSB and unfritted Ag paste (marketed by the Cermalloy Division ofHeraeus Incorporated of West Conshohocken, Pa., under Catalog No.C4400UF). The paste was printed onto both sides of the pellet to formelectrodes, and then the pellet was fired at 800° C. for 10 minutes tosinter the electrolyte without densifying it beyond the point necessaryto provide a robust structure and form a stabilized sensor cell withporous composite electrodes (about 18% porosity). Then Ag wire wasattached to both electrodes with a contact of Ag epoxy which was firedat 730° C. for 2 minutes. It is noted that thecomposite-electrode/electrolyte structure needs to be stabilized by theapplication of heat, pressure or chemical action, as the particularcomposition of the composite constituents may require or allow, in orderto provide a physically robust and stable product.

[0052] In the resulting composite electrode structure, we found that thedensification of the Ag phase is inhibited by the ion electrolytematerial phase. The electrodes then retain a porous microstructureduring and after thermally demanding processing and operation. Theretained porous microstructure significantly enhances the performance ofthe electrodes. In addition, electrodes of this kind have betteradhesion to the electrolytes because the stress arising from thermalexpansion mismatch between the ceramic electrolyte and the metalelectrode is minimized not only by the porous, heterogeneousmicrostructure of the electrodes but also by tailoring the thermalexpansion coefficient of the nanocomposites. We found that ananostructured electrode composite sintered to 50-85 percent of fulltheoretical density (i.e., producing an electrode composite with 15-50percent porosity) is optimal to obtain these advantages of performance.

[0053] Further, another important advantage is derived from usingnanocomposite electrodes. If the phases added are ionic conductors ormixed electronic-ionic conductors, the nanocomposite electrode as awhole turns out to be a mixed conductor, which allows ambipolartransport within the solid phase. FIG. 8 illustrates Nyquist plots ofimpedance spectra of cells with the nanocomposite electrodes incomparison with a cell with pure Ag electrodes. As determined from theimpedance spectra, the polarization resistances of the nanocompositeelectrodes are significantly smaller than that of the pure Ag electrode.As expected, the resistance of the nanocomposite electrode is a functionof the composition, i.e., the volume fraction of each constituent phase.This allows much room for performance optimization by adjusting thecomposition of the composites. The nanocomposite electrode shows anearly 4-fold reduction in electrode resistance as compared to the pureAg electrode. We found that good results are obtained with a mix of 0 toabout 65 vol % electrolyte (35 to 100 vol % metallic electrodematerial), the limit being that a continuum metal phase must exist for aviable porous electrode structure. That is, the amount of electrolytemust not be so great as to cause interruptions in the connectivity ofthe metal phase. At least 5 vol % electrolyte, 21 vol % being preferred,produced good results with different metal/electrolyte combinations. Webelieve that with composition optimization, further reduction inelectrode resistance can be achieved, leading to a significantenhancement in ion conducting device's performance.

[0054] A sensor produced with the structure manufactured in Example 3was operated in the following configuration, using air as the referencegas:

air, 79v%Ag21v%YSB|YSB|79v%Ag21v%YSB, Analyte gas.

[0055] The sensor response to the changes in the gas composition and intemperature was monitored by measuring the cell voltage under differentconditions.

[0056] Shown in FIG. 9 is the sensor voltage response in oxygen as afunction of temperature in comparison with that of a conventional YSZsensor. FIG. 9 clearly shows that the response of the YSB sensor followsNernst behavior down to 400° C., while the response of the YSZ sensordeviates from Nernst behavior below 500° C. This indicates that the YSBproof-of-concept sensor can be operated at a temperature about 100° C.lower than conventional YSZ sensors.

[0057]FIG. 10 further shows electrode resistances of nanocompositeelectrodes as a function of temperature, as determined from theimpedance spectra measured in air. Also shown in the figure are the datafor pure Ag electrode for comparison. As compared with pure Agelectrode, the nanocomposite electrodes show significantly lowerresistances. The ionic conductivity of the nanostructured YSBelectrolyte was measured and found to be over two orders of magnitudehigher than that of YSZ electrolyte, as shown in FIG. 11.

[0058] The impedance measurements and the data shown in these examplesestablish that nanostructured solid ion electrolytes and electrodes areindeed significantly superior in performance to solid ion electrolytesand electrodes prepared from micron-sized powders. The invention reducedto practice the use of nanostructured ion-conducting electrolytesexhibiting ion conductivity higher than obtained by prior-arttechnology; and it demonstrated the successful fabrication ofion-conducting electrolytes in general, and oxygen-conductingelectrolytes in particular, from materials with grain size less than 100nm for electrochemical, electrosynthesis and electrocatalyticapplications. It is noted that the methods of assembly or deposition ofnanoparticles to form structures according to this invention may varydepending on the particular application. For example, dry particles maybe pressed into a structure of predetermined geometry, as illustrated inExample 1, or deposited over a supporting substrate according to knownvapor deposition processes, as described in copending Ser. No.08/730,661 and illustrated in FIG. 12.

[0059] The process of deposition may also be incorporated with theprocess of manufacture of the nanosize particles disclosed in thereferenced copending applications. This method is preferred because itenables the continuous fabrication of product from raw material. Athermal reactor system is used to produce nanoscale powders byultra-rapid thermal quench processing of high-temperature vapors througha boundary-layer converging-diverging nozzle. A gas suspension of themicron-sized material is continuously fed to a thermal reaction chamberand vaporized under conditions that minimize superheating and favornucleation of the resulting vapor. The high temperature vapor isquenched by passing the vapor stream through the nozzle immediatelyafter the initial nucleation stages, thereby rapidly quenching itthrough expansion at rates of at least 1,000° C. per second, preferablygreater than 1,000,000° C. per second, to block the continued growth ofthe nucleated particles and produce a nanosize powder suspension ofnarrow particle-size distribution. A gaseous boundary-layer stream ispreferably also injected to form a blanket over the internal surface ofthe nozzle to prevent vapor condensation in the throat of the nozzle. Areceiving substrate is placed in the diverging section of the nozzle toreceive the nanoparticles produced in the quenched stream. Thus, ananostructured layer of electrolyte material can be deposited directlyas desired on the particular device being manufactured. As those skilledin the art would readily understand, the precise location of thesubstrate within the nozzle, the residence time, and other operatingparameters could be manipulated to produce the physical structuredesired for a particular application.

[0060] Potential applications of the invention include nanostructuredsolid electrolyte and electrode based devices for energy storage andgeneration such as, but not limiting to batteries, fuel cells, devicesfor thermodynamic property measurements; electrochemical sensors formonoatomic, diatomic and polyatomic gases such as, but not limiting toatomic oxygen found in atmosphere, diataomic oxygen and ozone sensors;ion sensors; oxygen pumps; solid state chemical pumps; monitors forsteam electrolyzers; measurement of dissolved oxygen in liquid metals;measurement of pH; electrocatalysis, electrosynthesis, catalyticmembrane reactors, and high-temperature kinetic studies. Therefore,while the present invention has been shown and described herein in whatis believed to be the most practical and preferred embodiments, it isrecognized that departures can be made therefrom within the scope of theinvention, which is therefore not to be limited to the details disclosedherein but is to be accorded the full scope of the claims so as toembrace any and all equivalent apparatus and methods.

We claim:
 1. A process of deposition comprising: providing a feedcomprising from the group consisting of solids and fluids; providing athermal reactor system to produce a high temperature vapor from thefeed; providing a zone to nucleate nanoscale powders from the hightemperature vapor; providing a thermal quench of said nucleated powdersat rates of at least 1000° C. per second through a converging-divergingnozzle; providing a substrate in the diverging section of the nozzle;and forming a nanostructured layer over the substrate.
 2. The process ofclaim 1 wherein the layer consists of an electrolyte.
 3. The process ofclaim 1 wherein the layer consists of an electrode.
 4. The process ofclaim 1 wherein the temperature of the high temperature vapor is greaterthan 1500 K.
 5. The process of claim 1 wherein the temperature of thehigh temperature vapor is greater than 3000 K.
 6. The process of claim 1wherein the nanoscale powders comprise an oxygen containing compound. 7.The process of claim 1 wherein the nanoscale powders comprise a metalcontaining compound.
 8. The process of claim 1 wherein the nanoscalepowers comprise a metal.
 9. A device prepared using the process ofclaim
 1. 10. A sensor prepared using the process of claim
 1. 11. A fuelcell prepared using the process of claim 1
 12. A battery prepared usingthe process of claim
 1. 13. A film prepared using the process ofclaim
 1. 14. A process for forming a nanostructured layer on a substratecomprising: feeding a raw material to a thermal reactor system, whereinthe raw material is a solid or a fluid; producing a high temperaturevapor from the raw material fed into the thermal reactor system and azone to form nucleated vapor from the high temperature vapor; quenchingthermally the nucleated vapor through a converging-diverging nozzle toform a nucleated nanoscale powder, wherein said quenching through thenozzle has a rate of at least 1000° C. per second; and providing asubstrate in a diverging section of the converging-diverging nozzle toform a nanostructured layer on the substrate.
 15. The process of claim14, wherein said nanostructured layer comprises an electrolyte.
 16. Theprocess of claim 14, wherein said nanostructured layer comprises anelectrode.
 17. The process of claim 14 wherein the temperature of thehigh temperature vapor is greater than 2500 K.
 18. The process of claim14 wherein the temperature of the high temperature vapor is greater than3000 K.
 19. The process of claim 14, wherein said nucleated nanoscalepowder comprises an oxygen containing compound.
 20. The process of claim14 wherein the nanoscale powders comprise a metal containing compound.21. The process of claim 14 wherein the nanoscale powders comprises ametal.
 22. A nanoscale film formed by the process of claim
 14. 23. Adevice comprising the nanoscale film of claim
 22. 24. A sensorcomprising the nanoscale film of claim
 22. 25. A fuel cell comprisingthe nanoscale film of claim
 22. 26. A battery comprising the nanoscalefilm of claim
 22. 27. A method for forming a nanostructured coatingcomprising: vaporizing a liquid precursor; delivering the vaporizedliquid precursor to a boundary-layer converging diverging nozzle,thereby forming a stream comprising of nanopowders; and delivering thestream comprising of nanopowders to a substrate, thereby forming ananostructured coating on the substrate.
 28. The method of claim 19wherein the step of vaporizing the liquid precursor is performed at atemperature greater than 2500 K.